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Sat, 07 May 2022 06:10:50 -0700 (PDT) Date: Sat, 7 May 2022 21:10:47 +0800 From: Muchun Song To: Mike Kravetz Cc: corbet@lwn.net, akpm@linux-foundation.org, mcgrof@kernel.org, keescook@chromium.org, yzaikin@google.com, osalvador@suse.de, david@redhat.com, masahiroy@kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, duanxiongchun@bytedance.com, smuchun@gmail.com Subject: Re: [PATCH v9 4/4] mm: hugetlb_vmemmap: add hugetlb_optimize_vmemmap sysctl Message-ID: References: <20220429121816.37541-1-songmuchun@bytedance.com> <20220429121816.37541-5-songmuchun@bytedance.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: C95141C0060 X-Stat-Signature: 5rtbsg6i98d85d7rktouxu1eus5rq3je X-Rspam-User: Authentication-Results: imf21.hostedemail.com; dkim=pass header.d=bytedance-com.20210112.gappssmtp.com header.s=20210112 header.b=6QjFTrBq; spf=pass (imf21.hostedemail.com: domain of songmuchun@bytedance.com designates 209.85.210.182 as permitted sender) smtp.mailfrom=songmuchun@bytedance.com; dmarc=pass (policy=none) header.from=bytedance.com X-HE-Tag: 1651929045-804906 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri, May 06, 2022 at 09:50:53AM -0700, Mike Kravetz wrote: > On 5/5/22 19:49, Muchun Song wrote: > > On Thu, May 05, 2022 at 09:48:34AM -0700, Mike Kravetz wrote: > >> On 5/5/22 01:02, Muchun Song wrote: > >>> On Wed, May 04, 2022 at 08:36:00PM -0700, Mike Kravetz wrote: > >>>> On 5/4/22 19:35, Muchun Song wrote: > >>>>> On Wed, May 04, 2022 at 03:12:39PM -0700, Mike Kravetz wrote: > >>>>>> On 4/29/22 05:18, Muchun Song wrote: > >>>>>>> +static void vmemmap_optimize_mode_switch(enum vmemmap_optimize_mode to) > >>>>>>> +{ > >>>>>>> + if (vmemmap_optimize_mode == to) > >>>>>>> + return; > >>>>>>> + > >>>>>>> + if (to == VMEMMAP_OPTIMIZE_OFF) > >>>>>>> + static_branch_dec(&hugetlb_optimize_vmemmap_key); > >>>>>>> + else > >>>>>>> + static_branch_inc(&hugetlb_optimize_vmemmap_key); > >>>>>>> + vmemmap_optimize_mode = to; > >>>>>>> +} > >>>>>>> + > >>>>>>> static int __init hugetlb_vmemmap_early_param(char *buf) > >>>>>>> { > >>>>>>> bool enable; > >>>>>>> + enum vmemmap_optimize_mode mode; > >>>>>>> > >>>>>>> if (kstrtobool(buf, &enable)) > >>>>>>> return -EINVAL; > >>>>>>> > >>>>>>> - if (enable) > >>>>>>> - static_branch_enable(&hugetlb_optimize_vmemmap_key); > >>>>>>> - else > >>>>>>> - static_branch_disable(&hugetlb_optimize_vmemmap_key); > >>>>>>> + mode = enable ? VMEMMAP_OPTIMIZE_ON : VMEMMAP_OPTIMIZE_OFF; > >>>>>>> + vmemmap_optimize_mode_switch(mode); > >>>>>>> > >>>>>>> return 0; > >>>>>>> } > >>>>>>> @@ -60,6 +80,8 @@ int hugetlb_vmemmap_alloc(struct hstate *h, struct page *head) > >>>>>>> vmemmap_end = vmemmap_addr + (vmemmap_pages << PAGE_SHIFT); > >>>>>>> vmemmap_reuse = vmemmap_addr - PAGE_SIZE; > >>>>>>> > >>>>>>> + VM_BUG_ON_PAGE(!vmemmap_pages, head); > >>>>>>> + > >>>>>>> /* > >>>>>>> * The pages which the vmemmap virtual address range [@vmemmap_addr, > >>>>>>> * @vmemmap_end) are mapped to are freed to the buddy allocator, and > >>>>>>> @@ -69,8 +91,10 @@ int hugetlb_vmemmap_alloc(struct hstate *h, struct page *head) > >>>>>>> */ > >>>>>>> ret = vmemmap_remap_alloc(vmemmap_addr, vmemmap_end, vmemmap_reuse, > >>>>>>> GFP_KERNEL | __GFP_NORETRY | __GFP_THISNODE); > >>>>>>> - if (!ret) > >>>>>>> + if (!ret) { > >>>>>>> ClearHPageVmemmapOptimized(head); > >>>>>>> + static_branch_dec(&hugetlb_optimize_vmemmap_key); > >>>>>>> + } > >>>>>>> > >>>>>>> return ret; > >>>>>>> } > >>>>>>> @@ -84,6 +108,8 @@ void hugetlb_vmemmap_free(struct hstate *h, struct page *head) > >>>>>>> if (!vmemmap_pages) > >>>>>>> return; > >>>>>>> > >>>>>>> + static_branch_inc(&hugetlb_optimize_vmemmap_key); > >>>>>> > >>>>>> Can you explain the reasoning behind doing the static_branch_inc here in free, > >>>>>> and static_branch_dec in alloc? > >>>>>> IIUC, they may not be absolutely necessary but you could use the count to > >>>>>> know how many optimized pages are in use? Or, I may just be missing > >>>>>> something. > >>>>>> > >>>>> > >>>>> Partly right. One 'count' is not enough. I have implemented this with similar > >>>>> approach in v6 [1]. Except the 'count', we also need a lock to do synchronization. > >>>>> However, both count and synchronization are included in static_key_inc/dec > >>>>> infrastructure. It is simpler to use static_key_inc/dec directly, right? > >>>>> > >>>>> [1] https://lore.kernel.org/all/20220330153745.20465-5-songmuchun@bytedance.com/ > >>>>> > >>>> > >>>> Sorry, but I am a little confused. > >>>> > >>>> vmemmap_optimize_mode_switch will static_key_inc to enable and static_key_dec > >>>> to disable. In addition each time we optimize (allocate) a hugetlb page after > >>>> enabling we will static_key_inc. > >>>> > >>>> Suppose we have 1 hugetlb page optimized. So static count == 2 IIUC. > >>>> The someone turns off optimization via sysctl. static count == 1 ??? > >>> > >>> Definitely right. > >>> > >>>> If we then add another hugetlb page via nr_hugepages it seems that it > >>>> would be optimized as static count == 1. Is that correct? Do we need > >>> > >>> I'm wrong. > >>> > >>>> to free all hugetlb pages with optimization before we can add new pages > >>>> without optimization? > >>>> > >>> > >>> My bad. I think the following code would fix this. > >>> > >>> Thanks for your review carefully. > >>> > >>> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c > >>> index 5820a681a724..997e192aeed7 100644 > >>> --- a/mm/hugetlb_vmemmap.c > >>> +++ b/mm/hugetlb_vmemmap.c > >>> @@ -105,7 +105,7 @@ void hugetlb_vmemmap_free(struct hstate *h, struct page *head) > >>> unsigned long vmemmap_end, vmemmap_reuse, vmemmap_pages; > >>> > >>> vmemmap_pages = hugetlb_optimize_vmemmap_pages(h); > >>> - if (!vmemmap_pages) > >>> + if (!vmemmap_pages || READ_ONCE(vmemmap_optimize_mode) == VMEMMAP_OPTIMIZE_OFF) > >>> return; > >>> > >>> static_branch_inc(&hugetlb_optimize_vmemmap_key); > >>> > >> > >> If vmemmap_optimize_mode == VMEMMAP_OPTIMIZE_OFF is sufficient for turning > >> off optimizations, do we really need to static_branch_inc/dev for each > >> hugetlb page? > >> > > > > static_branch_inc/dec is necessary since the user could change > > vmemmap_optimize_mode to off after the 'if' judgement. > > > > CPU0: CPU1: > > // Assume vmemmap_optimize_mode == 1 > > // and static_key_count == 1 > > if (vmemmap_optimize_mode == VMEMMAP_OPTIMIZE_OFF) > > return; > > hugetlb_optimize_vmemmap_handler(); > > vmemmap_optimize_mode = 0; > > static_branch_dec(); > > // static_key_count == 0 > > // Enable static_key if necessary > > static_branch_inc(); > > > > Does this make sense for you? > > Yes, it makes sense and is require because hugetlb_optimize_vmemmap_pages() > performs two functions: > 1) It determines if vmemmap_optimization is enabled > 2) It specifies how many vmemmap pages can be saved with optimization > hugetlb_optimize_vmemmap_pages returns 0 if static_key_count == 0, so this > would cause problems in places such as hugetlb free path (hugetlb_vmemmap_alloc). I hope my understanding is correct? > Right. > Would it make the code more clear if we did not do the check for > vmemmap_optimization in hugetlb_optimize_vmemmap_pages()? Instead: > - hugetlb_optimize_vmemmap_pages ALWAYS returns the number of vmemmap pages > that can be freed/optimized > - At hugetlb allocation time (hugetlb_vmemmap_free) we only check > hugetlb_optimize_vmemmap_enabled() to determine if optimization should > be performed. > - After hugetlb_vmemmap_free, we can use HPageVmemmapOptimized to determine > if vmemap pages need to be allocated in hugetlb freeing paths. > I think this works as well. My initial consideration was that embedding hugetlb_optimize_vmemmap_enabled() in hugetlb_optimize_vmemmap_pages() could make the caller (e.g. flush_free_hpage_work()) of hugetlb_optimize_vmemmap_pages() more efficient when static_key == 0. Maybe I could add the check for vmemmap_optimization to flush_free_hpage_work() and then remove the check from hugetlb_optimize_vmemmap_pages(). Will do this in a new version. Thanks. > Perhaps, there is something wrong with the above suggestion? > > I know you have always had hugetlb_optimize_vmemmap_pages perform the two > functions. So, splitting functionality may not be more clear for you. I am > OK leaving code as is (key inc/dec for each page). Just wanted to get your > (and perhaps other) thoughts on splitting functionality as described above. > -- > Mike Kravetz >